Organolithium compounds, e.g., butyllithium, are known to be produced by preparing a lithium or lithium/sodium metal dispersion in an inert organic liquid, and then adding a suitable alkyl halide which reacts with the lithium metal to form the organolithium product.
Generally, methyllithium was prepared by reacting chloromethane with two equivalents of lithium metal giving the product and lithium chloride.CH3Cl+2Li→CH3Li+LiCl
Classically, this reaction was performed in pure diethylether(DEE) as the solvent. However, unlike other alkyllithiums such as butyllithium, sec-butyllithium and tert-butyllithium, methyllithium is insoluble in pure hydrocarbon solvents. Diethyl ether, however, presents a serious potential for fire and explosions and is not a desirable solvent for industrial-scale reactions. Tetrahydrofuran (THF) is a more desirable ether because of it lower vapor pressure and lower autoignition temperature, but MeLi is unstable in pure THF. Methyllithium reacts with an equivalent of THF which then undergoes an irreversible fragmentation reaction. 
U.S. Pat. Nos. 4,976,886 and 5,141,667 disclose a process for producing organometallic compositions by reacting an organohalide with a mixture of two metals, one being an alkali metal, and the other being selected from magnesium, calcium, barium and zinc in a hydrocarbon solvent containing 0.5 to 2.0 moles of a Lewis Base per mole of organohalide. Methyltetrahydrofuran is disclosed a possible Lewis Base. The '667 patent also claims a method of preparing a stable methyllithium solution by adding a methyl halide to a mixture of lithium metal and an aromatic hydrocarbon containing tetrahydrofuran (THF) in an amount not exceeding 2 moles of THF per mole of methyl halide while maintaining the mixture at a temperature not exceeding 50° C. to react in an inert atmosphere the lithium metal and methyl halide to produce methyllithium and by-product lithium halide.
It has now been discovered that use of MeTHF with an aromatic solvent yields methyllithium compositions which are more stable than corresponding THF preparations when prepared as described herein.